The purpose of this proposal is to elucidate the role of visual attention in allowing the human visual system to achieve a variety of goals that must be attained if the content of a scene is to be analyzed successfully. The studies will focus on the influence of attentional manipulations on various perceptual performance measures in human subject with normal vision. The results obtained will lead to a greater understanding of the attentional requirements of normal vision, contributing to the comprehension of numerous visual and attention- related disorders. The project aims to determine whether there is a measurable attentional requirement to the perception of a variety of elementary features that are currently thought to be perceptible without the use of attention. The studies will investigate the types and extent of processing undergone by visual stimuli that receive little or no attention. The roles of preattentive and attentional processes in the perception of global and local characteristics of visual scenes will also be determined. These noninvasive experiments will be conducted with the participation of observers with normal or corrected-to-normal vision, who are naive as to the purposes of the experiments. Measures of accuracy in response to brief presentations, and reaction time to continuous displays, will be recorded in conditions differing by the attentional state of the observer, and by the attributes of individual items or scene organization.